ADP1073

–8–

REV. 0

Capacitor Selection

For optimum performance, the ADP1073’s output capacitor

must be carefully selected. Choosing an inappropriate capacitor

can result in low efficiency and/or high output ripple.

Ordinary aluminum electrolytic capacitors are inexpensive, but

often have poor Equivalent Series Resistance (ESR) and

Equivalent Series Inductance (ESL). Low ESR aluminum

capacitors, specifically designed for switch mode converter

applications, are also available, and these are a better choice

than general purpose devices. Even better performance can be

achieved with tantalum capacitors, although their cost is higher.

Very low values of ESR can be achieved by using OS-CON

capacitors (Sanyo Corporation, San Diego, CA). These devices

are fairly small, available with tape-and-reel packaging and have

very low ESR.

The effects of capacitor selection on output ripple are demon-

strated in Figures 12, 13 and 14. These figures show the output

of the same ADP1073 converter, which was evaluated with

three different output capacitors. In each case, the peak switch

current is 500 mA and the capacitor value is 100

µF. Figure 12

shows a Panasonic HF-series radial aluminum electrolytic. When

the switch turns off, the output voltage jumps by about 90 mV

and then decays as the inductor discharges into the capacitor.

The rise in voltage indicates an ESR of about 0.18

Ω. In

Figure 13, the aluminum electrolytic has been replaced by a

Sprague 593D-series device. In this case the output jumps

about 35 mV, which indicates an ESR of 0.07

Ω. Figure 14

shows an OS-CON SA series capacitor in the same circuit, and

ESR is only 0.02

Ω.

Figure 12. Aluminum Electrolytic

Figure 13. Tantalum Electrolytic

Figure 14. OS-CON Capacitor

If low output ripple is important, the user should consider using

the ADP3000. This device switches at 400 kHz, and the higher

switching frequency simplifies the design of the output filter.

Consult the ADP3000 data sheet for additional details.

All potential current paths must be considered when analyzing

very low power applications, and this includes capacitor leakage

current. OS-CON capacitors have leakage in the 5

µA to 10 µA

range, which will reduce efficiency when the load is also in the

microampere range. Tantalum capacitors, with typical leakage

in the 1

µA to 5 µA range, are recommended for very low power

applications.

Diode Selection

In specifying a diode, consideration must be given to speed,

forward voltage drop and reverse leakage current. When the

ADP1073 switch turns off, the diode must turn on rapidly if

high efficiency is to be maintained. Schottky rectifiers, as well as

fast signal diodes such as the 1N4148, are appropriate. The

forward voltage of the diode represents power that is not deliv-

diodes are recommended. Leakage current is especially impor-

tant in low current applications, where the leakage can be a

significant percentage of the total quiescent current.

For most circuits, the 1N5818 is a suitable companion to the

µA to 10 µA

leakage and fast turn-on and turn-off times. A surface mount

version, the MBRS130T3, is also available. For applications

where the ADP1073 is “off” most of the time, such as when the

load is intermittent, a silicon diode may provide higher overall

efficiency due to lower leakage. For example, the 1N4933 has a

1 A capability, but with a leakage current of less than 1

µA. The

higher forward voltage of the 1N4933 reduces efficiency when

the ADP1073 delivers power, but the lower leakage may outweigh

the reduction in efficiency.

For switch currents of 100 mA or less, a Schottky diode such as

than 1

µA. A similar device, the BAT54, is available in an SOT-23

package. Even lower leakage, in the 1 nA to 5 nA range, can be

obtained with a 1N4148 signal diode.

General purpose rectifiers, such as the 1N4001, are not suitable

for ADP1073 circuits. These devices, which have turn-on times

of 10

µs or more, are too slow for switching power supply appli-

cations. Using such a diode “just to get started” will result in

wasted time and effort. Even if an ADP1073 circuit appears to

function with a 1N4001, the resulting performance will not be

indicative of the circuit performance when the correct diode is

used.